Electrochemical etching cell

ABSTRACT

An electrochemical etching cell ( 1 ) is proposed for etching an etching body ( 15 ) made at least superficially of an etching material. The etching cell ( 1 ) has at least one chamber filled with an electrolyte, and is provided with a first electrode ( 13 ), which at least superficially has a first electrode material, and with a second electrode ( 13′ ) which at least superficially has a second electrode material. Furthermore, the etching body ( 15 ) is in contact, at least region-wise, with the electrolyte. In this context, the first electrode material and the second electrode material are selected such that, after the etching, the etching body ( 15 ) is not contaminated and/or is not impaired in its properties by the electrode materials. In particular, the electrode materials are the same materials as the etching material. Also proposed is a method for etching an etching body ( 15 ) using this etching cell ( 1 ), the first and/or the second electrode ( 13, 13′ ) being used as a sacrificial electrode. The proposed etching cell is particularly suitable for etching silicon wafers in a CMOS-compatible production line.

The present invention relates to an electrochemical etching apparatus,particularly a CMOS-compatible etching apparatus for etching siliconwafers, as well as a method for etching an etching body according to thespecies defined in the independent claims.

BACKGROUND INFORMATION

Electrochemical etching apparatuses, for example, for producing poroussilicon or for introducing pores on the surface of silicon, are usuallycomposed of a 2-chamber system, between which a silicon wafer to beetched is clamped as a separating wall, and the two chambers beingelectrically coupled or connected to one another only by the wafer.Furthermore, electrodes, generally made of platinum, are usually placedin both chambers for the current supply. For example, such an etchingapparatus is already described fully and in its essential details byFujiyama et al in the U.S. Pat. No. 5,458,755.

However, in known etching apparatuses, the problem continually occursthat at least the anodically connected electrode is at least slightlycorroded and dissolved during operation, so that initially theelectrolyte, and via it the wafer to be etched, becomes contaminated bythe dissolved electrode material in the course of the etching process.However, in many cases, such contamination, e.g. by platinum in asilicon production, is not acceptable and impairs the etched wafer orthe etching body in its electrical or catalytic properties considerably.

Thus, in particular a silicon wafer, on or in which a layer of poroussilicon was produced using an electrochemical etching process and which,in so doing, was contaminated with platinum, is unsuitable for use in aCMOS production (CMOS=complementary metal-oxide semiconductor).

Proposals for solving this problem which are based upon a one-sidedmetallization of the wafer back side and the use of an only one-sidedetching device, the back side of the wafer to be etched forming themetal contact and only the front side being in connection with theetching medium or the electrolyte and, via it, with a platinumelectrode, are unsuitable because of the requisite back-sidemetallization to be applied and the necessary sequence steps inprocessing the wafer (oxidation, layer depositions, etc.), for whichthis metallization then stands in the way.

SUMMARY OF THE INVENTION

Compared to the related art, the electrochemical etching apparatus ofthe present invention for etching an etching body and the method of thepresent invention carried out with it having the characterizing featuresof the independent claims have the advantage that contamination-freeelectrochemical etching of an etching body is thereby permitted, atleast on the surface the etching body having an etching material to beetched or being made of it. This holds true in particular for producingporous silicon from a silicon wafer. Consequently, the etching body isnot impaired, particularly in its electrical, i.e. electronic orcatalytic properties, by this etching.

Avoidance of contamination and/or impairment, associated at leastpartially with the contamination, of the electrical or catalyticproperties of the etching body obtained after the etching is veryadvantageously achieved in that the material of the electrodes, whichare directly connected to the etching body via electrolytes in theetching apparatus, is in each case selected according to the material ofthe etching body. When working with the etching method of the presentinvention using such an etching cell, at least one of these electrodes,at least with its side facing the etching body, is very advantageouslyused as a sacrificial electrode.

Advantageous further developments of the present invention result fromthe measures indicated in the dependent claims.

Thus, it is particularly advantageous if the material of a firstelectrode and/or the material of a second electrode, which are connectedas cathode and anode, respectively, and are electrically connected tothe etching body via a suitable electrolyte, is the same material as theetching material of the etching body to be etched. In this context, itis sufficient if the etching body, at least on the surface, has theetching material to be etched or is made superficially of it, and if thefirst and/or second electrode, at least on the surface, has acorresponding electrode material or is made superficially of it.

Furthermore, it is very advantageous if the etching material of theetching body is at least weakly electrically conductive, siliconpreferably being used as etching material or a silicon wafer being usedas the etching body. It is also advantageous if the first electrodematerial and the second electrode material of the first and secondelectrode, respectively, is a CMOS-compatible material, and inparticular is not an element selected from the group platinum, gold,iridium, rhodium, palladium, silver or copper. Consequently, the etchingapparatus of the present invention is particularly suitable forproducing porous silicon on a silicon wafer, contamination of the waferwith silicon-foreign substances such as platinum or palladium beingprevented, for example, by the use of silicon electrodes.

In this context, understood by a CMOS-compatible material according tothe general usage in semiconductor technology is a material which doesnot negatively affect the electrical properties of a circuit producedwith it.

Accordingly, to be understood by a material contaminating the etchingbody is, in particular, a CMOS poison or a material which, given itsembedment, forms deep imperfections in the etching body, that is to say,imperfections whose energy levels lie in the middle of the gap betweenconduction band and valence band of the material to be etched and whichconsequently give rise to a high transition matrix element for therecombination of electrons and holes in the etching body (“recombinationseed”).

Advantageously suitable as electrode materials for the first and secondelectrodes, respectively, are especially compounds from the group of theat least weakly conductive compounds of the elements silicon, carbon,nitrogen, oxygen, titanium, aluminum, boron, antimony, tungsten, cobalt,tellurium, germanium, molybdenum, gallium, arsenic and selenium,particularly SiC, SiN, TiN, TiC, MoSi₂ und GaAs, as well as pureelectrode materials from the elements silicon, titanium, tungsten,molybdenum and carbon, particularly graphite.

Generally, the materials commonly used as contact materials insemiconductor technology are also suitable as electrode materials, sincethey do not diffuse deeply into the etching body upon striking it, andtherefore during the etching, but rather react superficially with theetching body, i.e. are locally bound forming, for example, silicides,and thus remain restricted on the surface of the etching body. Thus, inthis sense they do not contaminate the etching body and also do notimpair it in its electronic properties, particularly with respect to theuse in, or compatibility with, a CMOS-compatible production line.

Advantageously, in each case, the respective electrode material isspecifically selected taking into consideration the material of theetching body and the electrolytes used. In addition, the first electrodeand/or the second electrode and/or the etching body are advantageouslyplanar, particularly in the form of wafers, the electrodes for use assacrificial electrodes very advantageously being substantially thickerthan the actual etching body besides, so that if desired, they can berecovered, freed from contamination and reused. The exchange cycles ofthe electrodes are thereby advantageously prolonged.

The electrochemical etching cell is advantageously constructed such thata first chamber and a second chamber are provided which are each filledat least partially with an electrolyte and which are separated spatiallyfrom each other via a separating device. Each of the two chambers iselectroconductively connected via an electrolyte to an electrode, theetching body at least region-wise being the separating device, and atthe same time very advantageously also being the only, at least weaklyconductive electrical connection between the two chambers and theelectrodes connected as cathode and anode, respectively.

A further very advantageous embodiment of the invention provides that,in addition to the two chambers already mentioned, the electrochemicaletching cell is provided with a further third chamber, or a furtherthird chamber and a further fourth chamber, which are each at leastpartially filled with an electrolyte and in each case are spatiallyseparated from the first chamber and second chamber, respectively, via afurther separating device. In this case, the electrolyte in the thirdand fourth chamber, respectively, is very advantageouslyelectroconductively connected only to the second and first electrode,respectively, which in turn simultaneously serve at least region-wise asthe separating device between the third and fourth chamber and the firstand second chamber, respectively.

In this connection, it is particularly advantageous if, only with theirsurface facing the etching body, are the especially planar first and/orthe second electrode in contact with the electrolyte that is in contactwith the etching body, so that the electrolyte in the third and fourthchamber is prevented from mixing with the electrolyte in the first andsecond chamber, respectively. Therefore, for simpler electricalcontacting of the electrodes, the side of the first and/or secondelectrode facing away from the electrolyte of the first or secondchamber can be provided superficially at least region-wise with ametallization or a doping or, for example, in the case when theelectrode is composed of a plurality of layers, can be made of a metal,which combines the advantage of a simple constructional design of theetching cell with the purposeful adaptation of the electrode material tothe respective etching material without contacting or contaminationproblems occurring.

Furthermore, for easy electrical contacting of the first and secondelectrodes via the respective electrolyte, provision can be made in thethird and fourth chamber, respectively, for an additional bathelectrode, particularly a platinum or palladium electrode, dipping intoan electrolyte located there.

Incidentally, the electrolytes in the individual chambers of the etchingapparatus according to the present invention can advantageously also bedifferent from one another, the first and second chambers in which theactual etching of the etching body takes place being advantageouslyfilled with hydrofluoric acid or a mixture of hydrofluoric acid andethanol, and the third and fourth chambers, for example, being filledwith diluted sulfuric acid as contact electrolyte.

Furthermore, the individual chambers are very advantageously capable ofbeing filled separately with electrolyte and emptied separately, thusallowing a problem-free exchange, for example, of a contaminatedelectrolyte in each chamber at any time. Consequently, in addition asimple exchange of an exhausted or contaminated first and/or secondelectrode used as a sacrificial electrode is made possible easily andquickly at any time.

Incidentally, the first and/or second electrode is advantageouslyelectrically contacted via the electrolyte, filled into the third andfourth chamber, respectively, to a bath electrode located there, andthus is connected to an external voltage supply which impresses acurrent on the etching apparatus during operation.

Moreover, the problem-free exchangeability of the sacrificialelectrodes, i.e. the first and/or the second electrode, veryadvantageously makes it possible, in a simple manner, to investigate thesuitability of different electrode materials such as graphite, forexample, during the etching of an etching body, and in so doing, tooptimize the electrode materials to the respective material of theetching body.

In addition, to homogenize the etching of the etching body in theetching apparatus according to the present invention, a tunnel ofnon-conductive material, particularly polypropylene, can advantageouslybe provided in a manner known per se.

BRIEF DESCRIPTION OF THE DRAWING

The present invention is explained in greater detail in the followingdescription with reference to the Drawing.

FIG. 1 shows a first electrochemical etching apparatus;

FIG. 2 shows an alternative specific embodiment of the etchingapparatus; and

FIG. 3 shows a third specific embodiment of the etching apparatus.

EXEMPLARY EMBODIMENTS

FIG. 1, as a first exemplary embodiment, shows an electrochemicaletching cell 1 of the present invention having four chambers, a firstchamber 19, a second chamber 19′, a third chamber 17 and a fourthchamber 18, each of which is filled at least partially with anelectrolyte. First and second chambers 19, 19′ are filled, for example,with a mixture of hydrofluoric acid and ethanol for the actual etchingof an etching body 15, while third and fourth chambers 17, 18 arefilled, for example, with diluted sulfuric acid as contact electrolyte.The four chambers 17, 18, 19, 19′ therefore define four electrolyteregions allocated to chambers 17, 18, 19, 19′, a first electrolyteregion 29, a second electrolyte region 29′, a third electrolyte region27 and a fourth electrolyte region 28 that are separated spatially fromone another via separating devices which at the same time, however,permit an electrical connection of chambers 17, 18, 19, 19′.

In detail, first chamber 19 is spatially separated from second chamber19′ via a first separating device 31, first chamber 19 is spatiallyseparated from third chamber 17 via a second separating device 32, andsecond chamber 19′ is spatially separated from fourth chamber 18 via athird separating device 33, so that no electrolyte is exchanged betweenchambers 17, 18, 19, 19′.

In this context, first separating device 31 is formed in a manner knownper se by an etching-body holding device 11 made of Teflon orpolypropylene, in which etching body 15 is fitted or insertedregion-wise so that it is in contact superficially with the electrolytein first chamber 19 on one side, and with the electrolyte in secondchamber 19′ on the other side. In the example explained, etching body 15is a generally known, planar silicon wafer. Second separating device 32and third separating device 33 are formed in each case by an electrodeholding device 10 made of Teflon, into which a second electrode 13′ anda first electrode 13, respectively, are inserted region-wise, so that atleast region-wise, they are superficially in contact with theelectrolyte of third and first chambers 17, 19′, respectively, on onehand, and with the electrolyte of second and fourth chambers 19′, 18,respectively, on the other hand.

As metallic contact electrode for contacting first and second electrodes13, 13′, respectively, in each case provision is made in third andfourth chambers 17, 18 for a platinum electrode or a palladium electrodeas bath electrode 34, 34′, which in each case dips into the electrolytelocated there. Bath electrodes 34, 34′ are further connected to avoltage source (not shown) which impresses an electric current onetching cell 1 in a generally known manner. In the explained example,relative to etching body 15, first electrode 13, i.e. its side facingetching body 15, is connected as anode, and second electrode 13′, i.e.its side facing etching body 15, is connected as cathode.

In the example explained, first electrode 13 and second electrode 13′are made of a planar silicon wafer or a silicon disk that is preferablysubstantially thicker than the silicon wafer used as etching body 15. Ingeneral, electrodes 13, 13′ are preferably selected with respect to theelectrode material used in each case, such that they are made at leastsuperficially of the same material as the respective surfaces of etchingbody 15. This ensures that the material of first electrode 13 and thematerial of second electrode 13′ do not contaminate etching body 15during operation of etching cell 1, and therefore do not impair itselectrical or catalytic properties after the etching.

During operation of etching cell 1, an external applied current nowflows across bath electrodes 34, 34′, the electrolytes, first and secondelectrodes 13, 13′ and etching body 15, etching body 15 being etched atleast superficially in a body etching region 14′. At the same time,however, depending on the selection of the material of electrodes 13,13′, first and second electrodes 13, 13′ are also etched, at leastsuperficially, in an etching region 14, that is to say, they are used assacrificial electrodes during the etching process of etching body 15. Inthis context, however, because of their markedly greater thicknesscompared to etching body 15, they are not etched through, but are merelysuperficially corroded, etched off, ablated or, for example, receivepores. When worn out, for example, after etching a plurality of etchingbodies 15, they can therefore be exchanged, recovered again or, ifnecessary, cleaned regularly of adhering contamination.

In detail, in the example explained, during the etching of a siliconwafer, porous silicon develops on its anodic side, i.e. in the case ofthe indicated polarity, in body etching region 14′, while at the sametime on the anodic side of first electrode 13 facing the etching body anat least slight etching likewise occurs in a corresponding electrodeetching region 14, that is to say, in the specific example, asuperficial formation of porous silicon. Incidentally, this also holdstrue for the side of second electrode 13′ facing away from etching body15, this electrode assuming the role of the anode in third chamber 17.At the same time, anodically connected metallic bath electrode 34 infourth chamber 18 also dissolves slightly during operation of etchingcell 1, however only the side of first electrode 13 facing away frometching body 15 being contaminated, for example, with platinum. However,because of the spatial separation of individual chambers 17, 18, 19, 19′between which only an electrical connection exists via electrodes 13,13′ and etching body 15, but between which no electrolyte exchange ispossible, this contamination stays away from etching body 15.Consequently, the contamination can be removed again from thecorresponding side during a recovery of electrodes 13, 13′.

Dissolving of silicon from one of electrodes 13, 13′ in the electrolytein first or second chamber 19, 19′ occurring, for example, during theetching, is not critical for etching body 15, since it is made of thesame material and therefore does not become contaminated.

Incidentally, for easy exchange of electrodes 13, 13′, they arepreferably joined to electrode holding devices 10 via seals, and arescrewed to etching apparatus 1 via closable windows 16 in side walls ofetching apparatus 1. A generally known quick-change fastener is furtherprovided for easy exchange of etching body 15.

In addition, for easy exchange of the electrolyte and electrodes 13, 13′used, chambers 17, 18, 19, 19′, i.e. associated electrolyte regions 27,28, 29, 29′ are each able to be filled and emptied separately by way ofsuitable, generally-known devices.

FIG. 2 clarifies a second exemplary embodiment of an etching cellaccording to the present invention. This etching cell is completelyanalogous to etching cell 1 according to FIG. 1 in essential points, buthas a different specific embodiment of the contacting of electrodes 13,13′. In this example, it is possible to dispense with third chamber 17and fourth chamber 18, bath electrodes 34, 34′ and the electrolytes inthese chambers 17, 18. Instead, first electrode 13 and second electrode13′ are each provided with a generally known metallization 20 on theside facing away from etching body 15.

Alternatively, however, electrodes 13, 13′ can also be provided with avery high doping on this side, thus ensuring good electricalconductivity. Finally, electrodes 13, 13′ can also be made of astratified body which has a metal layer on its side facing away frometching body 15, or is made of a metal. Further specific embodiments ofthe electrical contacting of electrodes 13, 13′ provide that, in amanner known per se, they are furnished with pin-, screen- or surfacecontacts on the side facing away from etching body 15 or, depending onthe electrode material, that electrodes 13, 13′ in first and secondchambers 19, 19′ are, particularly simply, partially immersed directlyin the electrolytes and are directly electrically contacted at alocation not immersed.

Thus, they are used as sacrificial electrodes instead of the platinumelectrodes known from the related art.

However, in the case of graphite as electrode material for first andsecond electrodes 13, 13′, contacting via an electrolyte in third andfourth chambers 17, 18, respectively, according to FIG. 1 isadvantageous.

Finally, reference should still be made to a third exemplary embodimentof the etching apparatus according to the present invention, clarifiedwith the aid of FIG. 3, in which, in contrast to FIG. 1, merely onegenerally-known tunnel 30 made of non-conductive material such aspolypropylene is additionally provided. This tunnel 30 is joined on bothsides to etching-body holding device 11 and concentrically surrounds a,for example, circular wafer as etching body 15. Tunnel 30 causeshomogenization of the flow lines in etching apparatus 1, and thus anexcellent thickness homogeneity of the etching of etching body 15,particularly when etching silicon to form porous silicon.

Further details of the above exemplary embodiments, which are known perse to one skilled in the art and are already fully explained, forexample, in U.S. Pat. No. 5,458,755, are dispensed with.

List of reference numerals

1 Etching cell

10 Electrode holding device

11 Etching-body holding device

12 Metal electrode

13 First electrode

13′ Second electrode

14 Electrode etching region

14′ Body etching region

15 Etching body

16 Window

17 Third chamber

18 Fourth chamber

19 First chamber

19′ Second chamber

20 Metallization

27 Third electrolyte region

28 Fourth electrolyte region

29 First electrolyte region

29′ Second electrolyte region

30 Tunnel

31 First separating device

32 Second separating device

33 Third separating device

34 Bath electrode

34′ Bath electrode

What is claimed is:
 1. An electrochemical etching cell for etching anetching body that is made at least superficially of silicon, comprising:at least two chambers filled at least partially with an electrolyte, oneof which is provided with a first electrode that at least superficiallyhas a first electrode material and the other of which is provided with asecond electrode that at least superficially has a second electrodematerial, wherein one of the electrodes is connected as a cathode andthe other of the electrodes is connected as an anode; wherein theetching body contacts at least region-wise with the electrolyte; andwherein the first electrode material and the second electrode materialinclude an at least weakly conductive compound including at least one ofsilicon, carbon, nitrogen, oxygen, titanium, aluminum, boron, antimony,tungsten, cobalt, tellurium, germanium, molybdenum, gallium, arsenic andselenium.
 2. The electrochemical etching cell of claim 1, wherein thefirst electrode material and the second electrode material include an atleast weakly conductive compound that includes at least one of SiC, SiN,TiN, TiC, MoSi₂ and GaAs.
 3. The electrochemical etching cell of claim1, wherein at least one of the first electrode material and the secondelectrode material is made of the same material as the etching material.4. The electrochemical etching cell of claim 1, wherein the etchingmaterial is at least weakly electroconductive.
 5. The electrochemicaletching cell of claim 4, wherein the etching material is silicon and theetching body is a silicon wafer.
 6. The electrochemical etching cell ofclaim 1, wherein the first electrode material and the second electrodematerial include CMOS-compatible materials.
 7. The electrochemicaletching cell of claim 1, wherein the first electrode material and thesecond electrode material do not include of any of platinum, gold,iridium, rhodium, palladium, silver and copper.
 8. The electrochemicaletching cell of claim 1, wherein at least one of the first electrode,the second electrode and the etching body is planar.
 9. Theelectrochemical etching cell of claim 8, wherein the surfaces of thefirst electrode and the second electrode facing the etching body are incontact at least region-wise with the electrolyte which is in contactwith the etching body.
 10. The electrochemical etching cell of claim 1,wherein the at least two chambers include a first chamber and a secondchamber, each of which is filled at least partially with an electrolyteand which are spatially separated from each other via a first separatingdevice.
 11. The electrochemical etching cell of claim 10, furthercomprising: a third chamber at least partially filled with anelectrolyte and spatially separated from the first chamber via a secondseparating device, the third chamber being electroconductively coupledto the second electrode and the second electrode at least region-wiseforming the second separating device.
 12. The electrochemical etchingcell of claim 11, further comprising: a fourth chamber at leastpartially filled with an electrolyte and spatially separated from thesecond chamber via a third separating device, the fourth chamber beingelectroconductively coupled to the first electrode, and the firstelectrode at least region-wise forming the third separating device. 13.The electrochemical etching cell of claim 10, wherein the first chamberand the second chamber are electroconductively coupled via the etchingbody.
 14. The electrochemical etching cell of claim 12, wherein at leastone of i) the first chamber and the third chamber areelectroconductively intercoupled via the second electrode and ii) thesecond chamber and the fourth chamber are electroconductivelyintercoupled via the first electrode.
 15. The electrochemical etchingcell of claim 10, wherein at least one of i) the first electrode isplanar and is electroconductively coupled only on one side to theelectrolyte of the second chamber, and ii) the second electrode isplanar and is electroconductively coupled only on one side to theelectrolyte of the first chamber.
 16. The electrochemical etching cellof claim 12, wherein at least one of i) the first electrode is planarand one side is electroconductively coupled to the electrolyte of thesecond chamber, with the other side electroconductively coupled to theelectrolyte of the fourth chamber and ii) the second electrode is planarand one side is electroconductively coupled to the electrolyte of thefirst chamber, with the other side electroconductively coupled to theelectrolyte of the third chamber.
 17. The electrochemical etching cellof claim 10, wherein at least one of the side of the first electrodefacing away from the electrolyte of the first chamber and the side ofthe second electrode facing away from the electrolyte of the secondchamber is one of made of metal, provided superficially, at leastregion-wise, with a metallization and provided superficially, at leastregion-wise, with a high doping.
 18. The electrochemical etching cell ofclaim 12, wherein at least one of the following is satisfied: the firstelectrode is coupled via the electrolyte in the third chamber to a bathelectrode; and the second electrode is coupled via the electrolyte inthe fourth chamber to the bath electrode.
 19. The electrochemicaletching cell of claim 13, wherein the bath electrode is a platinumelectrode.
 20. The electrochemical etching cell of claim 12, wherein thechambers are filled with different electrolytes.
 21. The electrochemicaletching cell of claim 20, wherein the first and second chambers arefilled with one of hydrofluoric acid and a mixture of hydrofluoric acidand ethanol, and the third and fourth chambers are filled with dilutedsulfuric acid.
 22. The electrochemical etching cell of claim 1, whereina tunnel made of non-conductive material is provided to homogenize theetching of the etching body.
 23. The electrochemical etching cell ofclaim 22, wherein the tunnel is made of polypropylene.
 24. Theelectrochemical etching cell of claim 22, wherein the chambers arefillable and emptiable separately.
 25. The electrochemical etching cellof claim 1, wherein at least one of the first electrode and the secondelectrode is used as a sacrificial electrode.
 26. The electrochemicaletching cell of claim 1, wherein during etching, at least region-wise,at least one of the first and the second electrode receives pores on thesurface.
 27. The electrochemical etching cell of claim 1, wherein theelectrodes are considerably thicker than the etching body.
 28. Theelectrochemical etching cell of claim 1, wherein the cell is used inetching silicon wafers in a CMOS-compatible production line.